This invention relates to induction furnaces used in the melting or smelting of metals and particularly to induction furnaces used in steelmaking.
In recent years there have been moves in the steelmaking industry to develop new steelmaking processes that are radically different compared to the traditional iron blast furnace and steelmaking-furnace routes.
In the traditional route steel is basically produced in two stages. In the first stage, which occurs in the blast furnace, iron oxide is reduced to pig iron. In the second stage, which occurs in the steelmaking furnace, elements such as carbon and manganese are controlled to specific levels and elements such as silicon, sulphur and phosphorous are mostly eliminated. Steelmaking furnaces include furnaces such as basic oxygen and electric arc furnaces.
One of the problems with the traditional method of making steel is the need to transfer liquid iron between the two stages of the process. The transfer involves a costly capital investment in infrastructure and also carries with it the risk associated with transporting liquid iron. The traditional methods are also associated with gas emissions that are not environmentally friendly.
A significant development in this area has been the development of a channel type induction furnace that is charged with an iron-containing burden and produces crude steel. This is the type of process described in U.S. Pat. No. 5,411,570 and patent applications PCT/EP97/01999 and PCT/IB99/01334.
The furnace is a channel type induction furnace and consists of a shell lined with refractory material. Feed material, iron containing ore and carbon reductant, is charged through holes in the sides of the furnace and is then heated by combustion of the different gases that are formed when a carbon reductant and ore mixture is heated, and under certain conditions, combustion of additional fuel.
Induction heaters situated at the bottom of the metal bath heat the liquid metal in the furnace which in turn heats the burden further and melts it to form liquid slag and metal. These heaters are attached to the furnace in the conventional manner. This means that the furnace has appropriate openings in its shell and flanges around the opening for bolting the complementary flange of the induction heater to the flange of the shell. Both the furnace and the induction heaters are lined with refractory material.
The thickness of the refractory material of the furnace around the induction heater opening in the furnace determines the depth of the entrance or xe2x80x98throatxe2x80x99 to the induction heater.
Molten metal flows into the induction heater through the throat and also exits the induction heater through it. The metal closest to the inner surface of the induction heater is heated. This means colder metal flows into the induction heater channels on the outside and is heated as it passes against the inside of the channel. Flow of the molten metal is generated by the difference in densities between hot and cold metal. Electromagnetic forces can assist this effect, to modify the flow pattern of the molten metal.
The known channel induction heaters are of the type that consists of an electrical coil that is built into a refractory body with a channel formed in the refractory material around the coil. The coil is isolated from the channel by refractory material, water-cooling panel(s) and an air gap. The combined depths of the refractory material on the floor of the furnace; the thickness of the furnace shell; the thickness of the furnace flange; and the distance between the furnace shell and the furnace flange is commonly accepted as the depth of the throat to the induction heater. The throat is shaped to be substantially vertical and it leads directly into the channels of the induction heater.
In the channel type furnace several of the induction heaters are arranged in a row along the length of the furnace.
The charge in the furnace consists of the molten metal bath, a layer of slag on top of the metal and the solid burden at the top. The burden is basically divided into two continuous heaps extending for the greater part of the length of the furnace, as described in U.S. Pat. No. 5,411,570; or the furnace can be charged so that the two continuous heaps of burden meet in the centre of the of the furnace to close the gap between the two heaps of burden, as described in patent application PCT/EP97/01999.
The molten metal flows into an induction heater through its throat and also exits the induction heater through its throat. The exit stream from the induction heater is substantially vertical, thereby mixing with the metal directly above the opening. The colder metal drawn into the induction heater also substantially originates from the pool of metal directly above the induction heater. The rising hot metal exchanges heat with the descending cold metal in the throat.
This means that the pool of metal above each induction heater opening and in the throat is to a large degree circulated through the induction heater and repeatedly heated. This causes local hotspots above the induction heater openings, especially when the depth of the metal bath above the induction heater is shallow. This causes the metal in the induction heater to be heated to unnecessarily, and some times dangerously, high temperatures.
The existence of local hotspots is not ideal in this type of furnace for a number of reasons. The first is that hotspots cause some of the burden in the vicinity of the hotspot to be preferentially melted, resulting in underexposure of that material to the heat from the burning gasses relative to the part of the burden not preferentially melted. Areas of overexposure and areas of underexposure to the heat from the burning gasses therefore exist. This difference in exposure leads to excessive electrical energy consumption and under utilisation of the available energy for reduction in the burning gasses and the heated roof. It also results in heating of unreduced burden that is too fast, leading to gas evolution in the liquid steel and subsequent undesirable boiling action. The effect of this is that the power input through the induction heaters must be reduced and as a result the production rate decreases.
In this specification the term xe2x80x9cthroatxe2x80x9d shall mean the communication channel between the furnace and an induction heater in the floor of the furnace.
In this specification the term xe2x80x9cthroat depthxe2x80x9d shall mean the operatively and substantially vertical distance from the uppermost extremity of the throat to a centre line drawn through the length of a coil of an induction heater in the floor of the furnace.
In this specification the term xe2x80x9cservice lengthxe2x80x9d shall mean the length of the furnace that each induction heater is required to heat during use, which is the operatively and substantially horizontal distance from the mid-point between an induction heater and an adjacent induction heater to the mid-point between the induction heater and an oppositely adjacent induction heater or to the end of the furnace.
In this specification the term xe2x80x9cthroat lengthxe2x80x9d shall mean the horizontal distance from one side of the throat of an induction heater, across the channels and the coil of the induction heater to its other side; this distance is measured substantially parallel to the xe2x80x9cservice lengthxe2x80x9d of the induction heater.
In this specification the term xe2x80x9cthroat widthxe2x80x9d shall mean the distance between sidewalls of the throat and this distance is measured transverse to the xe2x80x9cthroat lengthxe2x80x9d.
In this specification the term xe2x80x9cinduction heater channel widthxe2x80x9d shall mean the approximate distance from one side wall of the induction heater channel to the opposite side wall, measured at the centreline of the induction heater and measured at right angles to the long axis of the induction heater.
In this specification the term xe2x80x9cconventional throat depthxe2x80x9d shall mean, for a conventional induction furnace used for a similar process than that of the invention, the combined thickness of the floor refractory, the furnace shell supporting the floor, the distance between the furnace shell and the furnace flange, the thickness of the furnace and induction heater flanges, the thickness of the packing between the furnace and induction heater flanges, the distance between the induction heater flange and the induction heater shell, the induction heater shell, and the thickness of the induction heater refractory material from the induction heater shell upper inside surface to a level parallel with a centre line through the induction heater coil.
It is an object of this invention to provide a throat for a channel type induction heated furnace that at least partly alleviates some of the problems mentioned above.
In accordance with this invention there is provided for an induction-heated furnace comprising a shell lined with refractory material;
the furnace having at least walls and a floor;
with at least one induction heater located in the floor of the furnace;
the induction heater communicating with the interior of the furnace through a throat;
the throat length being more than at least half of the service length of the induction heater.
There is also provided for an induction heated furnace to comprise a shell lined with refractory material;
the furnace having at least walls and a floor with at least one induction heater located in the floor of the furnace;
the induction heater communicating with the interior of the furnace through a throat; and
the throat width being not more than three times the induction heater channel width, such throat width being substantially less than the width of a conventional throat in an induction heated furnace.
There is also provided for an induction heated furnace to comprise a shell lined with refractory material;
the furnace having at least walls and a floor;
with at least one induction heater located in the floor of the furnace;
the induction heater communicating with the interior of the furnace through a throat; and
the throat depth being substantially more than the throat depth of a conventional induction heated furnace used for the same process.
There is also provided for an induction heated furnace to comprise a shell lined with refractory material;
the furnace having at least walls and a floor;
with at least one induction heater located in the floor of the furnace;
the induction heater communicating with the interior of the furnace through a throat;
the interior at least partly filled with liquid metal; and
the level of liquid metal in the furnace being substantially less than the level of liquid metal in a conventional induction heated furnace used for the same process.
There is also provided for the furnace to be a channel type furnace;
for the furnace to be used in the melting, alternatively smelting, of metals,
for the furnace to have at least one charge hole for burden, at least one tap hole, and at least one gas burner inside the furnace.
There is further provided for the furnace to be a channel type furnace;
for the furnace to be used in steelmaking;
for the furnace to have at least one charge hole for iron containing burden, alternatively iron containing burden and reducing material, at least one tap hole, and at least one gas burner inside the furnace.
There is further provided for the burden to be scrap metal, for the burden to include reducing material and for the burden to include other raw materials.
There is also provided for the throat to have at least one baffle above the centre of the induction heater;
for the baffle to be built into the side walls of the throat; and
for the baffle to direct the flow of molten metal through the throat.
There is further provided for the throat to have baffles spaced throughout the throat;
for the baffles to be built into the side walls of the throat; and
for the baffles to direct the flow of molten metal through the throat.
There is further provided for the baffles to be preferably wedge shaped with the apex of the wedge directed to the centre of the induction heater.
There is also provided for the central baffle to have a weir on its operatively upper surface and for the weir to extend above the level of molten metal in the furnace.
There is further provided for a conduit to extend through the baffle and for the conduit to be a cooling conduit.
A further feature of the invention provides for an induction heated furnace as above, wherein the throat comprises at least two molten metal transport channels, the first channel communicating with a first portion of the molten bath above the induction heater, and the second channel communicating with a second portion of the molten bath remote from the first portion of the molten bath.
There is further provided for the throat to comprise three molten metal transport channels, for the second and third molten metal channels to respectively communicate with second and third portions of the molten bath remote from the first portion of the molten bath, and for the first portion of the molten bath to be located between the second and third portions of the molten bath.
The invention further provides for the operatively upper end of the first channel to include a manifold, for the manifold to be connected with a plurality of manifold passages, and for the passages to communicate with the operatively upper region of the first portion of the molten bath.
There is also provided for the passages to extend through a raised portion of the furnace floor.
A still further feature of the invention provides for the first channel to operatively channel molten metal from the induction heater to the molten bath, and for the second and third channels to operatively channel molten metal from the molten bath to the induction heater.